1. Field of the Invention
The present invention relates to a throttle valve control system for an automotive engine which controls the opening of the throttle valve so that it corresponds to the displacement of an accelerator.
2. Description of Related Art
A throttle valve opening control system, such as that known from, for example, Japanese Laid-Open Patent Publication 59-153,945, typically has a means for electrically controlling the opening of a throttle valve so that it corresponds to the displacement or operated stroke of an accelerator pedal precisely.
However, according to this kind of throttle valve control, because the throttle valve opens in accordance with the displacement or operated stroke of the accelerator pedal, with a given characteristic, even when an air-fuel ratio is regulated to provide a lean fuel mixture for low fuel consumption, the fuel mixture tends to become overly lean when driving the vehicle or operating the throttle valve and, therefore, the air-fuel mixture is apt to burn unstably.
In more detail, when the air-fuel ratio is controlled so as to provide a lean fuel mixture and the throttle valve is quickly opened, accompanying an acceleration, a rapid increase of intake air is induced. Consequently, the fuel mixture temporarily becomes overly lean, due to a time lag between the increase of fuel mixture and the increase of intake air. The air-fuel mixture, therefore, may fail to burn.
Controlling the air-fuel ratio so as to provide a lean fuel mixture for all of the cylinders of an engine, for the purpose of improving fuel consumption, may have an adverse effect on the stability of fuel combustion in lower engine load operating ranges. Therefore, it is desired, for improved fuel consumption and stability of combustion, to provide a lean fuel mixture for some of the cylinders of the engine and a rich fuel mixture for others of the cylinders. Also, when the throttle valves are operated to open for all the cylinders with the same given characteristic relative to the displacement or operated stroke of the accelerator, the combustion stability may be adversely affected.